This invention relates to a developing device of the type in which a thin layer of toner is formed on a toner conveying member and to a one-component type dry color toner to be used in this developing device for the purpose of developing electrostatic images.
The dry developing systems utilized in electrostatic photography, electrostatic recording, etc. may be classified into two types: systems using a two-component type developing agent composed of toner particles and carrier particles and those using a one-component type developing agent containing no carrier particles. A system of the former-type allows to obtain images of satisfactory quality with a relative steadiness. However, it is apt to involve carrier deterioration and variation in the toner/carrier mixing ratio, so that with such a system there is little possibility of obtaining images of constant quality for a long period of time. Besides, it involves a bothersome maintenance work and a rather bulky apparatus. In view of these problems, more and more attention is being paid to the latter-type systems, which are free from these problems, i.e., those systems which use one-component type developing agent.
In a system of the latter type, toner (developing agent) is normally conveyed by means of at least one toner conveying member, an electrostatic image formed on an electrostatic image carrier member being turned into a visual image by means of the toner conveyed. In this case, the toner layer formed on the toner conveying member must be as thin as possible. This also applies to systems using a two-component type developing agent the carrier particle diameter of which is very small. Further, when a toner which exhibits a high electrical resistance is adopted in a system using a one-component type developing agent, in particular, the toner must be charged by means of the developing apparatus, so that the toner layer is required to be extremely thin. When this toner layer is rather thick, only those portions of the toner layer which are near the surface of the toner layer will be charged, resulting in an uneven charging of the toner layer.
In view of this, various means for controlling the thickness of the toner layer formed on the toner conveying member (toner layer thickness controlling means) have been proposed. A typical example of such controlling means may be the one using a doctor blade, which is situated in a position opposite to the toner conveying member. This doctor blade controls the thickness of the toner layer by exerting pressurizing force on the conveyed toner on the surface of the toner conveying member.
While conventional developing devices having such toner layer thickness controlling means as described above allow to obtain satisfactory images in the earlier stages of copying, they involve gradual deterioration in image quality including unevenness in image density. In the case of color copying, in particular, a continuous copying with toner being supplied will involve reduction in the amount of charge which the toner receives, so that a color tone of image obtained in later stage of copying is distinctly different from that of the early copied image.